Get the upper, bottom, rightmost and leftmost point of a pixel-perfect BitmapData collision - actionscript-3

Question

How can I get the upper, bottom, rightmost and leftmost point of a pixel-perfect BitmapData collision? This is my collision-detection code:
public static function checkCollision(object1:*, object2:*, debug:Boolean = false):Boolean{
var object1Rect:Rectangle = object1.getRect(stage);
var object2Rect:Rectangle = object2.getRect(stage);
var object1Point:Point = new Point(object1Rect.x, object1Rect.y);
var object2Point:Point = new Point(object2Rect.x, object2Rect.y);
var bitmapData1:BitmapData = new BitmapData(
object1Rect.width,
object1Rect.height,
true,
0
);
var bitmapData2:BitmapData = new BitmapData(
object2Rect.width,
object2Rect.height,
true,
0
);
var clr:ColorTransform = new ColorTransform();
if(debug)
clr.color = 0x00ff00;
bitmapData1.draw(object1, new Matrix(1, 0, 0, 1, -object1Rect.x, -object1Rect.y), clr);
bitmapData2.draw(object2, null, clr);
if(debug){
if(bmp1.stage)
stage.removeChild(bmp1);
bmp1 = new Bitmap(bitmapData1);
bmp1.x = object1Point.x;
bmp1.y = object1Point.y;
stage.addChild(bmp1);
if(bmp2.stage)
stage.removeChild(bmp2);
bmp2 = new Bitmap(bitmapData2);
bmp2.x = object2Point.x;
bmp2.y = object2Point.y;
stage.addChild(bmp2);
}
var bCollide:Boolean = bitmapData1.hitTest(
object1Point,
255,
bitmapData2,
object2Point,
255
);
if(!debug){
bitmapData1.dispose();
bitmapData2.dispose();
}
return bCollide;
}
And it works perfeclty fine. However, the code I use to detect the top hitpoint doesn't work properly. This is the code:
public static function getHitPoint(object1:*, object2:*):Point{
var point:Point = new Point();
var object1Rect:Rectangle = object1.getRect(stage);
var object2Rect:Rectangle = object2.getRect(stage);
var object1Point:Point = new Point(object1Rect.x, object1Rect.y);
var object2Point:Point = new Point(object2Rect.x, object2Rect.y);
var bitmapData1:BitmapData = new BitmapData(
object1.width,
object1.height,
true,
0
);
var bitmapData2:BitmapData = new BitmapData(
object2.width,
object2.height,
true,
0
);
bitmapData1.draw(object1, new Matrix(1, 0, 0, 1, -object1Rect.x, -object1Rect.y));
bitmapData2.draw(object2);
var bitmap1:Bitmap = new Bitmap(bitmapData1);
var bitmap2:Bitmap = new Bitmap(bitmapData2);
bitmap1.x = object1Point.x;
bitmap1.y = object1Point.y;
bitmap2.x = object2Point.x;
bitmap2.y = object2Point.y;
var bitmapOrigin:Point = new Point(object1Point.x, object1Point.y);
var bitmap2OriginLocal:Point = bitmap2.globalToLocal(bitmapOrigin);
var overlappingPixels:Vector.<uint> = bitmap2.bitmapData.getVector(
new Rectangle(bitmap2OriginLocal.x, bitmap2OriginLocal.y, object1Rect.width, object1Rect.height)
);
for(var i:String in overlappingPixels){
var index:uint = uint(i);
if(overlappingPixels[i] != 0){
point.x = (index % object1.width) + (bitmap2.x > bitmap1.x ? bitmap2.x : bitmap1.x);
point.y = (uint(index / bitmap1.height)) + (bitmap2.y > bitmap1.y ? bitmap2.y : bitmap1.y);
break;
}
}
return point;
}
I've got no idea why, but the getHitPoint() function sometimes returns the wrong coordinates. Can anyone please explain why that is? And how can I detect the bottommost, the leftmost and the rightmost hitpoint?
Edit
I now know why getHitPoint() sometimes returned a wrong value: point.y = (uint(index / bitmap1.height)) + (bitmap2.y > bitmap1.y ? bitmap2.y : bitmap1.y); should be point.y = (uint(index/bitmap1.width)) + (bitmap2.y > bitmap1.y ? bitmap2.y : bitmap1.y);
Edit 2
I found out how to get the bottom hitpoint:
public static function getHitPoint(object1:*, object2:*, direction:int = 0):*{
var point:Point = new Point();
var object1Rect:Rectangle = object1.getRect(stage);
var object2Rect:Rectangle = object2.getRect(stage);
var object1Point:Point = new Point(object1Rect.x, object1Rect.y);
var object2Point:Point = new Point(object2Rect.x, object2Rect.y);
var bitmapData1:BitmapData = new BitmapData(
Math.round(object1Rect.width),
Math.round(object1Rect.height),
true,
0
);
var bitmapData2:BitmapData = new BitmapData(
Math.round(object2Rect.width),
Math.round(object2Rect.height),
true,
0
);
bitmapData1.draw(object1, new Matrix(1, 0, 0, 1, -object1Rect.x, -object1Rect.y));
bitmapData2.draw(object2);
var bitmap1:Bitmap = new Bitmap(bitmapData1);
var bitmap2:Bitmap = new Bitmap(bitmapData2);
bitmap1.x = object1Point.x;
bitmap1.y = object1Point.y;
bitmap2.x = object2Point.x;
bitmap2.y = object2Point.y;
var bitmapOrigin:Point = new Point(object1Point.x, object1Point.y);
var bitmap2OriginLocal:Point = bitmap2.globalToLocal(bitmapOrigin);
var overlappingPixels:Vector.<uint> = bitmap2.bitmapData.getVector(
new Rectangle(bitmap2OriginLocal.x, bitmap2OriginLocal.y, object1Rect.width, object1Rect.height)
);
switch(direction){
case 0: //top
for(var i:String in overlappingPixels){
var index:uint = uint(i);
if(overlappingPixels[i] != 0){
point.x = (index % bitmap1.width) + (bitmap2.x > bitmap1.x ? bitmap2.x : bitmap1.x);
point.y = (uint((index)/bitmap1.width)) + (bitmap2.y > bitmap1.y ? bitmap2.y : bitmap1.y);
return point;
}
}
case 1: //right
// I still need this
case 2: //bottom
overlappingPixels.reverse();
for(var i:String in overlappingPixels){
var index:uint = uint(i);
if(overlappingPixels[i] != 0){
point.x = bitmap1.width - (index % bitmap1.width) + (bitmap2.x > bitmap1.x ? bitmap2.x : bitmap1.x);
point.y = (bitmap2.y + bitmap2.height > bitmap1.y + bitmap1.height ? bitmap1.y + bitmap1.height : bitmap2.y + bitmap2.height) - (uint(index/bitmap1.width));
return point;
}
}
case 3: //left
// I still need this too
}
return false;
}
I still need a way to get the left and rightmost hitpoints though

Answer 1

You don't need to do it like you're doing there. You can do it all within a single function, which returns everything back correctly. I've added comments to the below. Please take note of what I've changed, as when you're trying to do it as you're doing now, with the code you changed, it is impossible.
This works for any shape, any direction. It'll give you the exact X and Y of the collision.
Please do not make this into a static function. Put it into a global class and use a Singleton to manage it instead. Things start to go very badly wrong when you being using static functions and reference the stage.
Also, if you're going to be working with pixel values of less than 1 (ie 99.75), the below will need a bit of adapting to cater for that. I've assumed you're using whole pixels, given your Math.round usage.
/**
*
* #param object1
* #param object2
* #return
*/
private function getHitPoint(object1:*, object2:*):*{
var point:Point;
// X and Y where we hit
// do NOT change this to a stage location or it does NOT work
var object1Point:Point = new Point(object1.x, object1.y);
var object2Point:Point = new Point(object2.x, object2.y);
var bitmapData1:BitmapData = new BitmapData(
Math.round(object1.width),
Math.round(object1.height),
true,
0
);
var bitmapData2:BitmapData = new BitmapData(
Math.round(object2.width),
Math.round(object2.height),
true,
0
);
// Draw
bitmapData1.draw(object1, new Matrix(1, 0, 0, 1, -object1.x, -object1.y));
bitmapData2.draw(object2);
// Create BMP's
var bitmap1:Bitmap = new Bitmap(bitmapData1);
var bitmap2:Bitmap = new Bitmap(bitmapData2);
// Set X and Y and BMP
bitmap1.x = object1Point.x;
bitmap1.y = object1Point.y;
bitmap2.x = object2Point.x;
bitmap2.y = object2Point.y;
// BMP origin is the object1 X and Y
var bitmapOrigin:Point = new Point(object1Point.x, object1Point.y);
// Create a local version of the bitmap2 so we can see what is overlapping
var bitmap2OriginLocal:Point = bitmap2.globalToLocal(bitmapOrigin);
// Create a rectangle from what we now know
var rect:Rectangle = new Rectangle(bitmap2OriginLocal.x, bitmap2OriginLocal.y, object1.width, object1.height);
// The overlapping pixels are within the rectangle, so get them all
var overlappingPixels:Vector.<uint> = bitmap2.bitmapData.getVector(rect);
// Run through all the overlapping pixels until we find a colourful one
for (var i:uint = 0; i < overlappingPixels.length; i++ ) {
var index:uint = overlappingPixels[i];
// If the colour is not 0, we have found it
if(index != 0){
point = new Point();
// Basically, instead of using width and getting 100, we're working out how
// many pixles across the overlap is. The Vector doesn't tell us this, so we need to work it out
var overlappingWidth:uint = object1.width - Math.abs(bitmap2OriginLocal.x);
// The Y is object1.y, minus the local y, plus object1's width minus the X from the local
point.y = object1.y - bitmap2OriginLocal.y + uint(i / overlappingWidth);
// The X is the same as above, but % of the width
point.x = object1.x - bitmap2OriginLocal.x + (i % overlappingWidth);
// Found it, we're done
break;
}
}
// Only fires when you've got a collision that is less than 1 pixel from the width or height
// Just a fail safe
if (!point) {
point = new Point(object1.width, object1.height);
}
return point;
}
For context, my entire class is below which shows how I was using this function. You can copy/paste this class and it will work. It shows how you move sprites around the screen, once it finds a collision, then it works out where the collision took place.
This class is for absolute pixel perfect collision detection, including an example.
package kazo
{
import flash.display.Bitmap;
import flash.display.BitmapData;
import flash.display.Sprite;
import flash.events.Event;
import flash.geom.Point;
import flash.geom.Rectangle;
import flash.geom.Matrix;
/**
* ...
* #author KM
*/
public class TestCases2 extends Sprite
{
private var rect :Sprite;
private var circle :Sprite;
/**
*
*/
public function TestCases2()
{
addEventListener(Event.ADDED_TO_STAGE, init);
}
public function init(e:Event):void {
removeEventListener(Event.ADDED_TO_STAGE, init);
trace('Starting test case');
// Rectangle
rect = new Sprite();
// Circle
circle = new Sprite();
// Draw the rectangle. Center point must be TOP LEFT
// If you're using Flash Professional, place everything at 0,0 inside the MC
rect.graphics.beginFill(0xff0000);
rect.graphics.drawRect(0, 0, 100, 100);
rect.graphics.endFill();
// Draw the circle. Center point is TOP LEFT, so the X and Y of the circle need to be equal to the radius
circle.graphics.beginFill(0xffff00);
circle.graphics.drawCircle(50, 50, 50);
circle.graphics.endFill();
// Add them
addChild(rect);
addChild(circle);
// Position
rect.x = 225;
rect.y = 75;
// Position
circle.x = 225;
circle.y = 225;
// Frame loop
addEventListener(Event.ENTER_FRAME, frameFunc);
}
/**
*
* #param e
*/
private function frameFunc(e:Event):void {
// move them around
circle.y -= 2;
circle.x += 0;
rect.y += 1;
rect.x += 0;
// Check for collisions. If found, stop. Pass 'true' as the final param if you want it to draw to the screen
if (checkCollision(rect, circle)) {
var ref:Point = getHitPoint(rect, circle);
// Draws where the collision hit
var loc:Sprite = new Sprite();
loc.graphics.beginFill(0x000000);
loc.graphics.drawRect(0, 0, 10, 10);
loc.graphics.endFill();
addChild(loc);
loc.x = ref.x;
loc.y = ref.y;
trace(ref);
removeEventListener(Event.ENTER_FRAME, frameFunc);
}
}
/**
*
* #param _obj1
* #param _obj2
* #param _debug
* #return
*/
private function checkCollision(_obj1:Sprite, _obj2:Sprite, _debug:Boolean = false):Boolean {
// Draw the first item to bitmapdata
var bmd1:BitmapData = new BitmapData(_obj1.width, _obj1.height, true, 0);
// ..and the second
var bmd2:BitmapData = new BitmapData(_obj2.width, _obj2.height, true, 0);
// Now draw them
bmd1.draw(_obj1);
bmd2.draw(_obj2);
// If we're in debug, also add the bitmap to the stage so we can see where we are
if (_debug) {
var bmp:Bitmap = new Bitmap(bmd1);
bmp.x = _obj1.x;
bmp.y = _obj1.y;
addChild(bmp);
var bmp2:Bitmap = new Bitmap(bmd2);
bmp2.x = _obj2.x;
bmp2.y = _obj2.y;
addChild(bmp2);
}
// Hit test including alpha channel. Obj1 X/Y, Obj2 X/Y, alpha channel
var rtn:Boolean = bmd1.hitTest(new Point(_obj1.x, _obj1.y), 255, bmd2, new Point(_obj2.x, _obj2.y), 255);
// Dispose the bitmap data, we dont need it anymore
if (!_debug) {
bmd1.dispose();
bmd2.dispose();
}
// Return the boolean
return rtn;
}
/**
*
* #param object1
* #param object2
* #return
*/
private function getHitPoint(object1:*, object2:*):*{
var point:Point;
// X and Y where we hit
// do NOT change this to a stage location or it does NOT work
var object1Point:Point = new Point(object1.x, object1.y);
var object2Point:Point = new Point(object2.x, object2.y);
var bitmapData1:BitmapData = new BitmapData(
Math.round(object1.width),
Math.round(object1.height),
true,
0
);
var bitmapData2:BitmapData = new BitmapData(
Math.round(object2.width),
Math.round(object2.height),
true,
0
);
// Draw
bitmapData1.draw(object1, new Matrix(1, 0, 0, 1, -object1.x, -object1.y));
bitmapData2.draw(object2);
// Create BMP's
var bitmap1:Bitmap = new Bitmap(bitmapData1);
var bitmap2:Bitmap = new Bitmap(bitmapData2);
// Set X and Y and BMP
bitmap1.x = object1Point.x;
bitmap1.y = object1Point.y;
bitmap2.x = object2Point.x;
bitmap2.y = object2Point.y;
// BMP origin is the object1 X and Y
var bitmapOrigin:Point = new Point(object1Point.x, object1Point.y);
// Create a local version of the bitmap2 so we can see what is overlapping
var bitmap2OriginLocal:Point = bitmap2.globalToLocal(bitmapOrigin);
// Create a rectangle from what we now know
var rect:Rectangle = new Rectangle(bitmap2OriginLocal.x, bitmap2OriginLocal.y, object1.width, object1.height);
// The overlapping pixels are within the rectangle, so get them all
var overlappingPixels:Vector.<uint> = bitmap2.bitmapData.getVector(rect);
// Run through all the overlapping pixels until we find a colourful one
for (var i:uint = 0; i < overlappingPixels.length; i++ ) {
var index:uint = overlappingPixels[i];
// If the colour is not 0, we have found it
if(index != 0){
point = new Point();
// Basically, instead of using width and getting 100, we're working out how
// many pixles across the overlap is. The Vector doesn't tell us this, so we need to work it out
var overlappingWidth:uint = object1.width - Math.abs(bitmap2OriginLocal.x);
// The Y is object1.y, minus the local y, plus object1's width minus the X from the local
point.y = object1.y - bitmap2OriginLocal.y + uint(i / overlappingWidth);
// The X is the same as above, but % of the width
point.x = object1.x - bitmap2OriginLocal.x + (i % overlappingWidth);
// Found it, we're done
break;
}
}
// Only fires when you've got a collision that is less than 1 pixel from the width or height
// Just a fail safe
if (!point) {
point = new Point(object1.width, object1.height);
}
return point;
}
}
}